Method and device for fine filtering

ABSTRACT

A method for use of an oil-hydraulic fine filter of the type where the outlet side of the fine filter is designed to operate without overpressure, where the working fluid that flows from the fine filter in a depressurized state is pumped on by a second pump, and where the second pump is driven by the same driving device as the feed pump of the fine filter.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to International Application No.PCT/NO02/00190, filed on May 30, 2002, which claimed priority toNorwegian Application No. 20012862, filed on Jun. 8, 2001, thedisclosures of which are incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

[0002] This invention regards a method of ensuring stable fine filteringof liquids in e.g. oil-hydraulic installations. The invention alsoregards a device for carrying out the method.

[0003] It is well known that the presence of contaminants in the form ofe.g. metal and mineral particles in the working fluid in oil-hydraulicinstallations is may reduce the service life of the components of theinstallation significantly.

[0004] Such installations therefore normally comprise at least onefilter designed to filter out contaminants. A simple and inexpensivesolution is to provide the suction side of the pump with a relativelycoarse suction strainer and the return line to the reservoir of theinstallation with a return filter. However, with this filterpositioning, contaminants that are generated in the hydraulic pumps willnot be captured before they flow into valves and actuators. Inparticular when using more complex equipment such as servo valves, ahigh pressure full flow filter is normally placed between the valves andpump(s) of the installation.

[0005] An ordinary full flow filter is not capable of filtering out thesmallest of the contaminants in question. It has therefore beennecessary in order to overcome this situation, to carry out relativelyfrequent oil changes or perform fine filtration, also termed deepfiltration, of the working fluid.

[0006] Fine filters of the type mentioned here are, if used correctly,capable of filtering out sufficiently fine contaminants to allow theworking fluid to be considered of satisfactory value in terms ofcontaminants, after treatment. Fine filters may be equipped with a waterseparator.

[0007] In order to obtain a satisfactory result from fine filteringthrough filters of the type in question, the fluid fed to the finefilter must lie within a relatively narrow pressure range, so as to keepthe pressure drop across the filter constant. The fine filter works bestwithout overpressure on the outlet side of the filter. Thus the filteredworking fluid flows away from the filter by means of gravity. The flowrate through the filter must be constant and preferably in a laminarform.

[0008] According to prior art, two principal methods are employed inconnection with the use of fine filters. By a first method, workingfluid is pumped via a separate pump from the main reservoir of theinstallation to a fine filter. The excess and filtered fluid is returnedto the reservoir in a depressurized state. By a second method, a certainvolume of working fluid is drained to the fine filter from the pressureloop of the installation by means of e.g. a constant flow valve.

[0009] The working fluid may flow back to the reservoir in adepressurized state; however it is more common to use a separatereservoir for fine filtered fluid, from where the fluid is laterreturned by means of a separate pump to the main reservoir. The pumpsused with a separate reservoir may be level controlled in order to allowautomatic control of the treatment plant.

[0010] It is also known to use a reversible pump that will feed workingfluid to the fine filter when rotating in one direction, and transferthe working fluid between the reservoirs when rotating in the oppositedirection. Inevitably of necessity, this latter method will prolong thefiltration period.

[0011] The possible positioning of the fine filter in relation to theremaining components of the hydraulic installation is limited to aconsiderable degree by the fact that the return side of the fine filteris unpressurized. Installations according to prior art are not suitedfor use in mobile installations such as diggers and equipment on wheels.

[0012] In fine filter installations where the filtered working fluid isreturned to a separate reservoir in a depressurized state, experiencehas shown that deposits of contaminants and accumulation of water mayoccur after a period of use. The cleaning required results in periodicshutdowns of the installation.

[0013] The object of the invention is to remedy the disadvantages ofprior art.

[0014] The object is achieved in accordance with the invention by thecharacteristics stated in the description below and in the appendedclaims.

SUMMARY OF THE INVENTION

[0015] A double pump, a tandem pump, a pump with a feed part and a mainpart, alternatively another pump equipped with two separate pump parts,are driven by one drive motor. The suction side of the first pump, thefeed pump, is connected to the main reservoir of the installation, andin operation pumps is working fluid from the reservoir to a fine filter.Any excess volume from the delivery side of the first pump flows througha pressure control valve to the suction side of the second pump, wherethe fine filtered working fluid from the outlet of the fine filter alsoflows in.

[0016] Excess volume here means the difference between the volumedelivered by the first pump, and the volume that may pass though thefine filter without causing the pressure drop across the filter tobecome higher than specified.

[0017] Thus the pressure control valve maintains a constant pressuredrop across the filter. As the fine filter gets clogged, the flow ofworking fluid through the pressure control valve will increase.

[0018] In one embodiment, working fluid flows back to the main reservoirfrom the delivery side of the second pump at a high velocity, so as toprevent any deposits and water accumulation from occurring. In analternative embodiment, working fluid from the delivery side of thesecond pump is passed to a downstream hydraulic installation that mayalso include other pumps, filters, valves and actuators.

[0019] It may be advantageous always to feed a limited amount of workingfluid from the delivery side of the first pump to the suction side ofthe second pump in order to prevent the second pump from running dryand/or cavitating.

[0020] The following describes a non-limiting example of a preferredmethod illustrated in the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 shows a schematic circuit diagram for a fine filtertreatment plant in which cleaned working fluid is led back to thereservoir; and

[0022]FIG. 2 shows a schematic circuit diagram of the installation inFIG. 1, but here, the outlet/delivery side of the second pump isconnected to a downstream hydraulic installation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0023] In the drawings, reference number 1 denotes a fine filtertreatment plant for working fluid.

[0024] Working fluid flows from a main reservoir 2 via a pipe 4 to theinlet/suction side of a first pump 6. The pump 6 and a second pump 8 aredriven by a motor 10. Working fluid flows from the delivery side of thefirst pump 6 via pipe connections 12, 14 and 16 to a pressure reducingvalve 18 and to the inlet side of a fine filter 20 via a check valve 22.From the outlet of the fine filter 20 and the outlet of the pressurereducing valve 18, working fluid flows through pipes 24 and 26 to thesuction side of the second pump 8.

[0025] A limited amount of working fluid flows through the connection 28from the delivery side of the first pump 6 to the suction side of thesecond pump 8 for reasons of lubrication and cooling.

[0026] From the delivery side of the second pump 8, working fluid flowsback to the reservoir 2 via a pipe 30, possibly through a check valve32.

[0027] The fine filter 20 may be equipped with a differential pressureindicator 34 and a flow meter 34 a. All the components of theinstallation are of a type that is known per se.

[0028] When the pump 6 feeds a greater volume of working fluid to thefine filter 20 through the pipes 4,12 and 16 than that for which thefine filter 20 is dimensioned, the pressure reducing valve 18 willmaintain an accurate inlet pressure to the fine filter 20 throughopening the pipes 14 and 26 to flow. This means that the fine filter 20can be utilised in the best possible manner with regard to bothtreatment capability and service life.

[0029] Working fluid flows to the suction side of the second pump 8 fromthe fine filter 20 through the pipe 24 in a manner such that the outletside of the fine filter is not subjected to overpressure, and from thepressure relief valve 18 through the pipe 26. In the second pump 8, thepressure is increased in order to enable the fluid to be used in asubsequent operation, whereby the velocity of the working fluid may beincreased when flowing back to the reservoir 2 via the pipe 30, by meansof e.g. a restriction (not shown) in the pipe.

[0030] In an alternative embodiment, see FIG. 2, the pipe 30 isconnected to a downstream hydraulic installation 40 that may include oneor more of the following components: a pump 42 with a motor 44, a mainfilter 46, a check valve 48, a pressure relief valve 50 and valves andactuators 52. All these of a type that is known per se.

[0031] Thus the fine filter treatment plant 1 can take care of theentire supply of working fluid to the hydraulic installation 40.Alternatively, the pump 42 may through use of the motor 44 also pumpworking fluid to the hydraulic installation 40. The main filter 46 is afull flow pressure filter protected by the bypass coupled check valve48. The pressure relief valve 50 prevents overloading of valves andactuators 52.

[0032] By using the method according to the invention, one drive unitwill be sufficient for the pumps of the installation, both in the caseof pressurized installations and unpressurized installations. Returningthe working fluid to the reservoir 2 at a substantial velocity resultsin good circulation in the reservoir, whereby deposits are prevented.

[0033] Furthermore, it is an advantage that fine filtering according tothe proposed method is relatively insensitive to sudden movement andthus well suited for mobile equipment.

[0034] A further advantage is the freedom of placement of the finefilter in relation to the remaining components of the hydraulicinstallation, and the fact that there is no requirement for a separatefilter reservoir.

1. A method for use of an oil-hydraulic fine filter (20) of the typewhere the outlet side of the fine filter (20) is designed to operatewithout overpressure, characterized in that working fluid that flowsfrom the fine filter (20) in a depressurized state is pumped on by asecond pump (8), where the second pump (8) is driven by the same drivingdevice (1) as the first pump/feed pump (6) of the fine filter (20) andthat the first pump's/feed pump's (6) outlet communicates with thesecond pump's (8) in through a pressure reducing valve (18).
 2. A methodin accordance with claim 1, characterized in that working fluid from thesecond pump (8) flows to the reservoir (2).
 3. A method in accordancewith claim 1, characterized in that working fluid from the second pump(8) flows to a hydraulic installation (40).
 4. A method in accordancewith one or more of the preceding claims, characterized in that workingfluid flows from the delivery side of the feed pump (6) via a pressurerelief valve (18) to the suction side of the second pump (8).
 5. Amethod in accordance with one or more of the preceding claims,characterized in that a limited amount of working fluid flows from thedelivery side of the feed pump (6) to the suction side of the secondpump (8).
 6. A device by an oil-hydraulic fine filter (20) of the typewhere the outlet side of the fine filter (20) is designed to operatewithout overpressure, characterized in that the outlet side of the finefilter (20) is connected to the suction side of the second pump (8), andwhere the second pump (8) is operated together with the feed pump (6) ofthe fine filter (20), by the same driving device (1), and where thefirst pump's/feed pump's (6) outlet is connected with the second pump's(8) inlet through a pressure reducing valve (18).